When Winston Smith secretly purchases a diary and begins recording his private thoughts (a special class of crime known as thoughtcrime), he is worried the thought police will discover it and begin monitoring him. Consequently, he decides to place a speck of dust on the cover of the diary when he puts it away. He assumes the thought police will either not notice the dust or not consider it significant. If Winston looks at the diary and the dust is missing, he will know the police have been there. The dust never goes missing, so Winston believes his diary is a secret. Unfortunately, the thought police have been reading his diary; contrary to Winston's plan, the thought police recognized the significance of the dust and carefully replaced it each time they read his diary.